The present invention relates to a data transmission device, a data transmission system and a method for activating a data transmission and, in particular, to a device and a method of a wake-up signaling and a simultaneous network setup.
In order for several participants (e.g. radio nodes) to be able to transmit data via a network, it is, for example, standard practice to use a time-controlled media access method. Network protocols for wireless networks in this respect frequently use TDMA methods (TDMA=time division multiple access). The radio nodes of such networks contain standard transceiver units. In TDMA methods, the transceivers are only activated at certain points in time and only for a short time. During this time, the communication is executed. Thereafter, the transceiver is switched off again (idle state or idle phase or sleep time). Thus, in particular the receiver is not active most of the time and thus not reachable for a radio communication (latency). In order to save current, the TDMA method is frequently operated with a low duty cycle, wherein the duty cycle defines, for example, the percentage of an overall period in which the transmission means transmits data. In this scenario, thus an increase in the reaction time or an unwantedly long latency results. An event may only be detected and passed on in the next receive time. In order to be able to acquire operation times in the order of years with small batteries (e.g. button cells), such a receiver of a standard transceiver generally has to be switched off for minutes (this also depends on the battery used) in order to then be able to be switched on again only for a short time. The duty cycle thus has to be very short.
Current-saving receivers (e.g. wake-up receivers) are thus set up very simply and comprise a moderate sensitivity. The achievable power consumption is only in an order of magnitude of 100 microamperes, however. Thus, they are suitable for steady-state operation, but may generally not be used as a main receiver as their sensitivity is too low.
Wireless networks usually maintain their synchronicity by regular synchronization messages (e.g. so-called beacons). To regularly receive the same, switching on the receiver at the right time (in the TDMA method) is needed. With very long sleep times between the phases in which a data transmission may take place, drift effects may have a negative effect. Depending on the accuracy of the internal timers, the times drift apart and the next synchronization message may be missed. This may be prevented by making sleep times shorter, which is a disadvantage in terms of energy, but additional synchronization mechanisms may bring an improvement. One possibility is additional synchronization messages which only serve for synchronization but do not enable a data transmission. Likewise, drift effects may only be balanced by longer receive windows.
Wireless sensor networks usually use TDMA-based protocols. With very current-saving applications, sleep times are very long and thus also latency. Thus, a long time passes until an event may be reported across the network. With battery-supported applications with an operating life time above 1 year, latency may already be in the range of minutes. With systems having a wake-up receiver, the same generally only serves for activating the main system in case of an event, like, for example, an interrupt. After activating, a network setup is executed by a new synchronization.
From the above-mentioned, for the described access methods a series of problems result for the network. Thus, for example, current-saving synchronous wireless sensor networks generally operate with the TDMA method so that they are not ready to receive continuously, but only at discrete points in time. Due to the non-continuous capacity to receive, the energy consumption is reduced, on the other hand, however, an event may only be reported or transmitted at discrete times via the network. The resulting latencies are the longer, the lower said duty cycle. This is not acceptable for many applications.
A further problem results from the limited operation time with battery operation. If the desired operation time with battery operation, for example in particular with button cell operation, is set to more than a year, a maximum admissible current consumption of the used receiver of clearly less than, for example, 100 μA results. Such an average current consumption may only be acquired using standard transceivers and TDMA methods if the duty cycle is in a unary or one-digit percentage. This, again, involves the disadvantage of a very long latency.
A further problem results from the fact that with very long sleep times which may, for example, be in a range of several hours, drift effects may have a negative effect. These drift effects describe a diverging of the timers for the individual network nodes if they have not been synchronized over a long period of time (i.e. were not adapted). In order to compensate this, additional measures for synchronization would be needed or the network nodes would have to comprise a longer receive window. The additional measures may, for example, include additional signals which are transmitted across the network. In any case, thus the energy consumption would rise again. Thus, a compromise between the most efficient use of available energy possible and preventing drift effects is needed. This compromise limits the energy-saving potential considerably.
Finally, it is disadvantageous in standard technology that for receiving synchronization signals generally the main receiver is used, which again leads to an increased current or energy consumption.